Gyrate atrophy of the choroid and retina (GA) is an autosomal recessive chorioretinal degeneration, characterized by night blindness in childhood and progressive loss of peripheral vision leading to blindness in the 4th or 5th decades. Affected patients have marked hyperornithinemia due to deficient activity of ornithine aminotransferase (OAT or OKT). OAT is a major catabolic enzyme for ornithine and requires pyridoxine (Vitamin B6) as a cofactor. Based on the results of vitamin B6 therapeutic trials and in vitro OAT assays, the majority of GA patients are classified as B6 nonresponsive while a few patients are B6 responsive. Many fibroblasts and/or lymphoblasts from GA patients of both types and from obligate heterozygotes are available for molecular investigation of the primary defect. The structural gene for OAT has been mapped to chromosome 10 and OAT related sequences are present on the X chromosome. Restriction fragment length polymorphisms at the OAT locus will be used to haplotype the defective alleles associated with GA. The data will aid in prenatal diagnosis and carrier detection of GA, but more importantly, will lay the groundwork for isolation and characterization of several independent mutant OAT alleles. cDNAs prepared using mRNA from representative GA lines will be characterized by sequencing and compared with the normal gene to identify the mutation. For GA cell lines that show absence of OAT mRNA or an abnormal DNA pattern, genomic clones will be isolated and characterized to locate the primary defect by comparison to structure of the normal OAT gene and by functional studies. Mutations in the leader peptide affecting the transport of OAT precursor into the mitochondria and the role of leader peptide in the transport will also be explored. The OAT cDNA will be expressed in cells deficient in OAT activity. The proposed studies should yield a detailed understanding of the various genetric defects that may cause GA. The information gained will be clinical application in the diagnosis of GA, in identification of carriers of the defective OAT allele for purposes of family counselling, and possibly in treatment of the disorder.